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Galaxy Collisions Give Birth to Quasars

Peering behind thick clouds of cosmic gas and dust, researchers believe they have finally determined the origins of quasars, the brightest and most powerful objects in the universe. X-ray and infrared observations of over 200 distant galaxies, coupled with images taken in visible light, reveal that quasars form when two galaxies smash into each other and their central black holes merge. The new observations also suggest that quasars were more common in the early universe than previously thought.

Astronomers discovered quasars, short for quasi-stellar objects, in the 1950s. Only about the size of our solar system, quasars easily outshine entire galaxies and can burn for 100 million years. For decades, however, astronomers could not figure out what generated these cosmic beacons. The obvious suspects were supermassive black holes, which anchor the cores of practically all galaxies, can devour gigantic amounts of matter, and are known to produce huge jets of particles and energy. But many galaxies—including the Milky Way—host supermassive black holes yet don't generate quasars.

Perhaps the younger quasars were hiding. At least, that's what astronomers began to suspect in the late 1990s when they noticed that some galaxies whose central cores were obscured by huge clouds of dust seemed to emit the same kind of radiation and produced similar levels of energy as quasars.

To peek behind the dust, astronomer Ezequiel Treister of the University of Hawaii, Honolulu, and colleagues first selected about 200 candidate galaxies out of 100,000 images taken by the Hubble Space Telescope. Then they trained the Chandra and Spitzer space telescopes, which can see in x-ray and infrared light, respectively, on the galaxies' obscured cores. The candidates ranged out to a distance of about 11 billion light-years—back to a time when the universe was only 2.7 billion years old. The new observations revealed hidden quasars in every one of the galaxies, the team reports online today in Science. What's more, by studying the shape of the galaxies in the Hubble images, the astronomers found that they all had arisen from the mergers of two massive galaxies and their central black holes.

Taken together, the data paint a picture of how quasars form. "Suddenly it all made sense," says astronomer and co-author Priyamvada Natarajan of Yale University. When two big galaxies merge, she explains, their supermassive black holes combine. The new black hole, which weighs as much as billions of suns, begins devouring everything in its vicinity. For up to 100 million years, this activity is shrouded by the dust kicked up by the galaxy merger. But eventually, the black hole consumes enough of the dust that the powerful and brilliant particle jets blasting out from its poles can break into the open. After another 100 million years, the quasar exhausts its fuel and goes dark, becoming just a garden-variety, billion-solar-mass or so black hole.

In the early universe, collisions—and hence quasars—occurred more often because galaxies were packed much more closely together. For many years, Treister says, astronomers doubted that obscured quasars were very common. "Now," he says, "we are seeing them everywhere."

The findings provide "powerful insights into how quasars form and evolve," says astronomer Amy Barger of the University of Wisconsin, Madison. The notion that obscured quasars are the byproduct of galaxy mergers has been around for quite a while, adds astronomer Christopher Reynolds of the University of Maryland, College Park. "But it is amazing that the fraction of obscured quasars tracks the galaxy-merger rate so well," he says. "It means that there isn't much room for other mechanisms to produce obscured quasars."